1.Field of the Invention
This invention relates to a apparatus and a method for manufacturing semiconductor single crystals by the Czochralski Method (hereinafter referred as the CZ method).
2.Description of the Related Art
The steps of manufacturing silicon single crystals aided by applying of magnetic field are: disposing electric magnets around a single-crystal manufacturing apparatus, applying a magnetic field on the melt to increase its dynamic viscosity, and growing silicon single crystals by the CZ method. Due to the fact that the convection of the melt is restrained by the action of the magnetic field, temperature variations at the location near the free surface of the melt are thus moderated, and a steady growing of silicon single crystals can be obtained. Furthermore, the reaction between the melt and the quartz crucible (SiO.sub.2) can be restrained or enhanced by the above steps, thereby providing an effective way to control the oxygen density in silicon single crystals.
FIG. 3 is a schematic drawing showing the operation of pulling up semiconductor single crystals aided by applying a magnetic field. FIG. 3 primarily shows the distribution of flux lines, depictions of components other than a quartz crucible 5 and two facing homopolar magnets 1 and 2 are omitted. Usually, in the case of pulling up silicon single crystals aided by applying a Cusp magnetic field, the free surface of the melt 6 becomes located at the central portion between the up-going magnetic field and the down-going magnetic field. Therefore, as shown by the dotted line in FIG. 3, above the free surface of the melt 6, the flux lines of the facing homopolar magnet 1 deviate in a substantially horizontal direction and penetrate the sidewall of the quartz crucible 5 in a normal direction. Similarly, below the free surface of the melt 6, the flux lines of the facing homopolar magnet 2 deviate into a substantially horizontal direction and penetrate the sidewall of the quartz crucible 5 in a normal direction. This can restrain the convection of the melt along the sidewall of the quartz crucible 5. However, the magnetic field strength at the location adjacent to the growth boundary of the single crystal approaches zero, therefore it is unable to restrict the convection of the melt occurring at the location adjacent to the growth boundary of the single crystal.
Concerning the process of manufacturing single crystals aided by applying a Cusp magnetic field, a method of pulling up single crystals has been disclosed in Japanese Patent Publication Gazette TOKU KOU HEI 2-12920. According to TOKU KOU HEI 2-12920, facing homopolar magnets are respectively disposed on the upper and lower portions of the outer wall of the single-crystal pulling up chamber, and equal-axially symmetric and radially distributed Cusp magnetic fields are formed within the melt during pulling up of single crystals. Furthermore, according to the method of growing single crystals disclosed in Japanese Patent Publication Gazette TOKU KOU HEI 1-282185, during the pulling up of single crystals, the free surface of the melt is located at the central portion between the up-going Cusp magnetic field and the down-going Cusp magnetic field. In addition to the above, several proposals concerning method of growing single crystals aided by applying a Cusp magnetic field have also been offered.
However, in the process of pulling up single crystals disclosed in Japanese Patent Publication TOKU KOU HEI 2-12920, the central portion between the up-going Cusp magnetic field and the down-going Cusp magnetic field is lowered into the interior of the melt during the pulling up operation. This will cause an increase in the viscosity of the melt located beneath the growth boundary of the single crystal, and there is a danger of causing uneven impurity distribution in the silicon single crystal. Furthermore, since the convection of the melt in the upward and downward directions occurring at the location beneath the growth boundary of the single crystal can not be restrained, the density of oxygen entering the single crystal will increase. Likewise, in the process of pulling up single crystals disclosed in Japanese Patent Publication TOKU KOU HEI 1-282185 or in application of the ordinary CZ method by applying a Cusp magnetic field as shown in FIG. 3, the free surface of the melt is located at the central portion between the up-going magnetic field and the down-going magnetic field. In these cases, the magnetic field distribution in the melt near the growth boundary of the crystal is the most dispersed, and the restriction on the convection near the free surface of the melt is small. Therefore, if debris of the quartz crucible or amorphous silicon will float on the free surface of the melt, and these impurities will enter the silicon single crystal being pulled up to cause polycrystallization of the single silicon crystal. Hence, it is difficult to pull up the silicon single crystal at an extremely low oxygen density.